IMPROVED DIRECT TUBE REPAIR PROCESS

Abstract: The wall of a pressure vessel tube having stress-corrosion cracks or the like is repaired by localized melting using laser welding, for fusing over defects by melting and re-solidifying the metal. A reactive cover gas is maintained over the area of localized melting in order to eliminate cracking caused by impurities in the tube material. The cover gas may be carbon dioxide or other gasses which act to scavenge the impurities. The wall of the tube may be melted twice in order to further improve the surface finish of the repair area; the first melt being to a depth of at least 80 % of the wall thickness, and the second melt being to a depth of about one-third the wall thickness.

Description: IMPROVED DIRECT TUBE REPAIR PROCESS BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This application is a continuation-in-part • of serial number 07/998,218, filed on December 30,1992. The invention relates to reforming degraded areas in ductile materials, in particular by melting a localized area to a predetermined depth, re-forming the localized area by cooling it, and advancing the localized melting and cooling through the degraded area to restore it to an integrally continuous form. The invention is particularly applicable to fusing service-induced stress and corrosion defects in coolant circuit tubes of pressurized water nuclear reactors. PRIOR ART The parent of this patent application, serial number 07/998,218 filed on December 30, 1992, describes a laser weld repair technique known as "Direct Tube Repair" or "DTR", both of which are trademarks of the Westinghouse Electric Corporation. The DTR process involves reforming portions of a heat exchanger tube wall by heating with laser energy. The DTR technique offers significant advan¬ tages over alternative plugging and sleeving repair meth¬ ods. However, laboratory and field experience with Direct Tube Repair processes has shown that under certain condi- tions, cracking can occur in the reformed sections of the tube wall. Furthermore, the reformed areas of the tube wall may not always be readily inspectable by conventional nondestructive examination techniques due to irregularities in the surface finish of the weld which may occur under certain conditions. Therefore, it is an object of this invention to provide a weld repair process which provides crack-free welds. It is a further object of this invention to provide crack-free weld repairs having a surface finish which facilitates conventional nondestructive examination tech¬ niques. These and other aspects of this invention are met in a method for repairing heat exchanger tubes having the steps of melting a localized area of the tube wall in the degraded area while maintaining a reactive cover gas over the localized area, and allowing the localized area to cool and solidify, thereby reforming the localized area. The cover gas may be carbon dioxide or a combination of carbon dioxide and oxygen. The repair method may be enhanced by a further step of remelting the localized area to a second depth which is substantially less than the depth of the original melt. The realization of these objects will be appreci¬ ated from the following discussion of particular exemplary embodiments of the invention. However it should also be appreciated that the invention is capable of variation from the examples, in accordance with the scope of the invention as claimed. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, FIGURE 1 is a partial section view showing application of the invention to the repair of heat exchang¬ er tubes in a nuclear steam generator plant; FIGURE 2 is a schematic illustration of a welding means for directing laser emissions against the inner walls of a tube to be repaired by localized melting of the tube along a scanned progressive pattern; FIGURE 3 is a schematic illustration of a method for relative displacement of the tube and welding means; FIGURE 4 is an elevation view, partly in section, showing application of an alloying agent in connection with the welding; FIGURE 5 is plan view of a tube inside surface following a direct tube repair as described; FIGURE 6 is a longitudinal section view through a weld line according to FIGURE 1; and, FIGURE 7 is a lateral cross section through an alternative form of repair using a consumable insert alloying material. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGURE 1, for repairing a wall 22 of a pressure vessel tube 24 having a deteriorated zone, a welding head 32 is placed into the tube 24 at the deterio- rated zone. The welding head is activated and moved progressively relative to the tube so as to melt a local¬ ized point along a line 42 on a surface of the wall. As the welding head is advanced, a welding line is formed, with the tube material behind the point of application of the weld head cooling and solidifying. The welding process melts and fuses the degraded area over a welding line having a width equal to the localized point of melting, and to a depth in the wall 22 defined by the dimensions over which the welding head applies energy, the amplitude of the energy applied and the time the energy is applied to a given location. The welding head is operated at a suffi¬ cient power level and is advanced at a sufficiently slow speed that the localized point is melted to a depth such that after solidifying the tube is restored to serviceable condition for its intended use. Solid material surrounds the localized area that is melted at any one time, and supports the melted material. After passage of the welding head, the surrounding solid material cools the material quickly by carrying away the thermal energy applied by the welding head. Any defects which were present in the degraded zone of the tube become fused due to the melting of the tube material. The weld melts the material of the tube at /03250 . PCMJ S94/09570 least to a depth equal to a part of a thickness of the wall. It is possible to melt entirely through the depth of the tube wall, because the melted volume is conical or cup- shaped in cross section, with the width of the melted portion being greatest at the radial inside of the tube, and less proceeding away from the weld head. The melted material cools upon passage of the weld head, whereupon a repair has been effected without the necessity of adding to the wall thickness, plugging the tube or otherwise adverse- ly affecting the flow and thermal characteristics of the tube. Continuously during melting along a first line, or stepwise after the weld line has passed over a predeter¬ mined length, the welding head 32 is displaced laterally of the first line. Localized melting is continued along a line which is adjacent or overlapping the first line to melt and cool, thus to reconstitute the degraded area over a further width adjacent the first weld line. The weld head is advance linearly and laterally in this manner, successively melting linear sections of the wall and fusing the wall over the entire degraded area in a raster-like series of passes. The weld line is preferably advanced laterally by an amount less than the width of the weld line 42, such that the first weld line and the further weld line partly overlap, and a part of the first weld line is remelted in the process of forming the next. The lateral advance can be stepwise or continuous and can involve any pattern of adjacent, preferably-over¬ lapping passes which encompass the whole area of the repair. One alternative is to rotate the welding head relative to the axis of the tube to form the welding line and axially to advance the welding head relative to the tube to form the further width. When advancing the line of welding continuously, this motion produces a helical pattern of weld lines as shown in FIGURES 1 and 2. Another alternative is to relatively displace the point of application of energy via the welding head and the tube axially in an oscillating motion to form the welding line. The welding head is also relatively rotated with respect to the tube to form the further width. The pattern produced by this motion is represented by FIGURE 3. The rotation can be stepwise, continuous or oscillating. Preferably, the welding process uses laser welding, although other means for isolated local melting of a point on the tube are also possible. For laser welding the welding head comprises an optical system 62, directing laser emissions onto the degraded area 26. Mirrors 64, lenses 66 and fiber optic light conduits 68 can be em¬ ployed. An example of an appropriate laser welding device for use according to the invention is disclosed in US Patent 4,694,136 - Kasner et al, which is hereby fully incorporated. Referring to FIGURES 1 and 2, a drive means 72 is operable to rotate and axially translate a stem 74 compris¬ ing the welding head 32. The fiber optic cable 68 couples the welding head to a high powered laser 76, for example a ND:YAG laser. The distal end 82 of the fiber optic cable is spaced from mirror 64. A first lens 66 collimates the light diverging from the end of the fiber optic cable and a second lens 67 focuses the light at the point of applica¬ tion to the tube wall. Lens 67 has a focal length substan¬ tially equal to the sum of the distances between lens 67 and the center of mirror 64, and between mirror and the point of welding. The light emitted from the fiber optic cable is thereby focused at a spot on the area 26 of tube 24 that is being repaired. The drive means 72 can rotate the stem relative to the fiber optic cable. Whereas the light is collimated between lenses 66 and 67, the axial position between end 82 and lens 66 is held constant, i.e., at the focal distance of the lens. The distance between lenses 66 and 67 can be varied, e.g., with axial displace¬ ment due to operation of the drive means 72. However, it is preferred in connection with axial displacement to move the welding head or stem axially as a unit to effect axial displacement . o FIGURES 1 and 2 illustrate an embodiment arranged to produce a helical pattern 48 of weld lines. In FIGURE 3 an axial pattern is produced, using an axially oscillat¬ ing drive means that moves the weld head up and down in the tube. A motor 96 can be provided for this purpose as shown. As in the previous embodiment, lenses focus the light emitted at the end 82 of the fiber optic cable 68. The welding head is advanced axially and rotation- ally to cover the entire deteriorated area 26, in a series of passes. Parallel axial weld lines as shown in FIGURE 3 can be made by rotationally indexing the weld head. Slanting or helical lines can be made by rotating the weld head continuously during scanning of the laser beam. In order to guide each weld line so as to evenly overlap the previous weld line, it is possible to vary the rate of advance (and perhaps focusing) of the laser beam on the workpiece. Preferably, each weld line is tracked relative to the position of a previous weld line. This can be accomplished by providing a guide on the welding head, operable to rest against a ridge or other dimensional variation at the edge of the last weld line. FIGURE 4 shows the surface appearance of the inside wall of a tube following a direct surface repair according to the invention. Each weld line in this case is placed adjacent the previous line, with a slight overlap, e.g., 50 to 80% of the width of the weld line. The specif¬ ic power level of the laser can be varied as needed to accommodate a desired area over which the laser is to be focused, and a desired rate of advance. An average power of at least 200 watts can be used for welding, and an average power of 200-800 watts can be used advantageously. The depth of the weld can be varied as a function of power level, focusing and rate of advance, in order to melt the tube material to the required depth. The tempera- ture of melting of course varies with the material of the tube. For Inconel 600 stainless steel (ASME Alloy 600), as advantageously employed for steam generator heat exchanger tubes, the melting temperature is about 1,350 to 1,410°C (or 2,470 to 2,575°F) . The typical thickness of the tube wall of a nuclear steam generator is about 0.050 to 0.055 inches (1.3 to 1.4mm). Preferably the weld depth extends through 80 to 100% of the wall thickness. Of course it is possible to apply the invention to thicker or thinner tubes or to materials other than stainless steel, by correspondingly changing the power level, the rate of advance of the beam, etc. The dimensions, power levels and the like are exem¬ plary only. FIGURE 5 shows an elevation view of an actual tube weld, including the partly overlapping weld lines. The surface of the inner surface of the tube is rendered somewhat less smooth due to the welds, however the inside diameter of the tube is only minimally reduced. As shown in FIGURE 6 via a longitudinal cross section through a line of welding, a shallow penetration surface repair by welding melts the tube through about 40% of its thickness. With the use of a narrow bead, the weld can extend through 100% of the tube thickness. This is possible because the bead tends to taper in cross section, having a typically conical shape as shown in FIGURE 7. Although the melted material extends through the wall, the lateral dimensions of the bead at the outer wall surface are relatively small. Accordingly, the unmelted portion of the tube mechanically supports the melted bead. The area which is melted at any one time is relatively small and does not tend to flow, making it possible using this technique to weld quite deeply into the tube. Additionally, the heat energy applied at the welding point is quickly carried away and the melted portion cools promptly after the welding head passes. An alloying material 54 can be diffused into the material of the tube during the welding process, and consumed. The alloying material can be applied as a powder that is sprayed or painted onto the tube surface, either before or during welding, for example together with appli¬ cation of a welding cover gas. The alloying material may also be applied as a sleeve shaped insert that is consumed in the process and fused with the melted and reformed material of the tube. The results of welding over an alloying material 54 are shown in a lateral cross section through a series of weld lines in FIGURE 6. In order to obtain good control of the depth of penetration of the weld repair, it has been found that a relatively slow laser pulse frequency should be maintained, with a relatively long pulse duration. For the typical Inconel 600 nuclear plant steam generator tubing described above, a pulse frequency of less than about 20 Hz and a pulse duration of above about .005 seconds are preferable, using a laser at about 300-325 watts of average power. Acceptable welds having 80-100% wall thickness penetration have been obtained using a pulsed YAG laser set for a pulse frequency of 14 Hz and a pulse duration of .0076 seconds at these power levels. These parameters provide a pulse shape with a high peak power during the initial portion of the energy pulse, and a lower energy level during the remainder of the pulse. For a given average power output, a rela- tively low frequency will result in a deeper penetration into the tube wall because of the effect of the power peak. As a result of the high peak power and low pulse frequency, the dominant cooling mechanism for the weld pool is radia¬ tion, and a portion of the weld pool returns to the solid condition between the energy pulses. Because conduction cooling is less dominant than radiation cooling under these conditions, the weld repair is less sensitive to heat sink conditions outside of the tube wall, such as the presence or absence of a tube support plate, tube sheet or moisture. It has been found that these parameters can provide crack- free weld repairs for Inconel tubing which has sulfur levels in the usual range of .002-.003 percent by weight. However, when the sulfur levels increase to about .004 percent, the resulting weld repairs remain subject to cracking. For Inconel tubing having sulfur content above about .004 percent, it is desirable to utilize laser energy parameters which result in conduction being the predominant cooling mechanism and which will maintain the weld pool as a liquid between energy pulses, in order to maximize the dispersion of the contaminants in the weld pool. For repairs of Inconel 600 nuclear steam generator tubing using the laser system described above, pulse frequencies of at least about 100 Hz and pulse durations of no more than about .001 seconds have been found to provide crack free weld repairs in such material . In order to obtain the desired 80-100% penetration of the tube wall with these parameters, the average power output of the laser must be higher due to the relatively lower power peak. A disadvan¬ tage of such parameters is that the repaired surface tends to be rippled, probably due to instabilities in the weld pool resulting from the higher power level, and this makes it difficult to perform nondestructive testing on the repaired tube wall areas. Furthermore, at these parameters the weld is more sensitive to heat sink variations outside of the tube wall because of the increased influence of conduction cooling. It is known in the art to apply an inert cover gas over a weld pool to isolate the weld from external contami¬ nants and oxygen. However, it has been found that reduced sensitivity to impurities in steam generator tube wall material may be obtained by utilizing a reactive cover gas which acts to scavenge the impurities from the weld pool. Carbon dioxide, a mixture of carbon dioxide and air, and a mixture of carbon dioxide and oxygen have all been used successfully. Anhydrous ammonia or a mixture of argon and hydrogen would provide a similar benefit. A reactive cover gas may be supplied to the weld area via the inside diameter of the weld head stem 74, as illustrated in Figure 3. The cover gas can be made to pass over the mirrors 64, lenses 66 or other optical components in order to protect them from -weld splatter and in order to provide cooling. Carbon dioxide supplied in this manner will provide adiabatic cooling as it expands within the weld head, thereby providing cooling to the optical compo¬ nents. Since the oxygen in the carbon dioxide is in combined form, it does not oxidize the optical components. As the carbon dioxide enters the weld zone the heat of the welding process breaks down the carbon dioxide, thereby releasing free oxygen which mixes with the molten metal. The free oxygen combines with the tramp elements, such as sulfur, and as a result it eliminates cracking of the weld metal upon cooling. This process may be augmented through the addition of controlled amounts of free oxygen in addition to the carbon dioxide. A second cover gas may be supplied to the weld area via the inside diameter of the tube under repair. In this manner, it is possible to provide a combination of two types of cover gases; a first cover gas being selected primarily for its cooling proper¬ ties and a second cover gas being selected primarily for its ability to scavenge impurities from the weld pool. For example, carbon dioxide may be provided over the optical components via the weld head stem, and air or oxygen may be provided along the inside of the tube. In spite of the beneficial effect of the special cover gasses, for some tube materials it may not be possi¬ ble to obtain crack-free laser weld repairs having an inspectable surface condition, while at the same time using process conditions which provide good control of the depth of weld penetration. In such situations, a two-step weld repair technique may be used. It has been found that cracks occurring in tube wall repair welds as a result of impurities such as sulfur in the weld pool are most often found in the upper portion of the weld, i.e. the surface closest to the heat input surface. Cracks in nuclear steam generator tubing resulting from sulfur levels in excess of 0.003 percent are typically found in the top one-third of the weld. By using a two step process, good control of the depth of the weld penetration can be maintained in the first step, then the top portion of the repair area can be remelted to eliminate any cracks resulting from the first step. For example, the laser energy parameters for the first step may be selected to have a relatively low pulse frequency and a relatively long pulse duration, thereby providing good control of the depth of penetration. In this manner a first repair having a weld depth of 80 to 100% of the wall thickness can be obtained with relatively low sensitivity to external heat sink conditions. For Inconel 600 nuclear steam generator tubing, a 14 Hz pulse rate and .0076 second pulse duration may be selected at an average power level of approximately 300-325 watts. If cracking occurs in the weld repair area as a result of the tubing having more than about .004 percent by weight of sulfur or other similarly acting contaminants, such crack¬ ing will be predominantly in the top portion of the weld. Such cracking can be subsequently repaired using a second heat cycle having laser energy parameters selected to be relatively high in frequency and relatively short in pulse duration, for example 100 hertz and .001 second respective¬ ly. With these parameters an average power level of about 300 watts will provide melting to a depth of only about a third of the tube wall thickness, since the peak power is reduced as a result of the increased frequency. This second step will provide a crack-free weld, and in the process, will repair any cracks resulting from the first melt. Another consequence of using only enough average power to obtain melting of about one third of the tube wall thickness is that the surface finish of the weld will be sufficiently smooth to perform routine nondestructive examinations, since the instabilities of the weld pool are minimized due to the shallow depth of the pool. This two step process can be implemented with a variety of cover gasses, such as those discussed above, to further minimize the cracking resulting from contaminants in the weld pool. The invention is particularly applicable to correcting degradation of the heat transfer tubes of a nuclear steam generator plant. Typically, a plurality of individual tubes 24 are arranged parallel to one another and extending between inlet and outlet manifolds, one wall 25 of a manifold being shown in FIGURE 1. Access to the tubes can be obtained from inside the manifolds, for example controlling the weld head by remote control and thus avoiding human exposure to the environment of the reactor systems. This invention may also be applied to any other type of tubular product, for example a pipe or a reactor vessel head penetration; and further, it may be applied to any part having a wall, for example a valve body, a tank wall, etc. The invention having been disclosed, a number of variations and alternatives will now be apparent to persons skilled in the art. The invention is not limited to the examples disclosed above and includes a reasonable extent of variation in accordance with the appended claims, to which reference should be made in assessing the scope of the invention in which exclusive rights are claimed.

Claims: CLAIMS: 1. A method for reforming a part with a wall having a thickness, the wall having a degraded area extend¬ ing into the thickness, the method comprising the steps of: melting a localized area of the wall to a first depth in the degraded area while maintaining a reactive cover gas over said localized area,* and allowing the localized area to cool and solidify, thereby reforming the localized area to said first depth. 2. The method of claim 1, wherein said cover gas comprises carbon dioxide. 3. The method of claim 1, wherein said cover gas comprises a mixture of carbon dioxide and oxygen. 4. The method of claim 1, wherein said part is a tube, and further comprising the step of melting said localized area by applying laser energy via a welding head inserted into said tube. 5. The method of claim 4, further comprising the step of providing a first cover gas to said localized area through said welding head. 6. The method of claim 5, further comprising the step of providing a second cover gas to said localized area via the inside diameter of said tube. 7. The method of claim 5, wherein said first cover gas comprises carbon dioxide, anhydrous ammonia, or a mixture of argon and hydrogen. 8. The method of claim 6, wherein said second cover gas comprises oxygen. 9. The method of claim 6, wherein said first cover gas comprises carbon dioxide and said second cover gas comprises oxygen. 10. The method of claim 1, further comprising the steps of: remelting said localized area of the wall to a second depth substantially less than said first depth; and allowing said localized area to cool and solidify. 11. The method of claim 10, wherein said first depth is at least 80% of said tube wall thickness, and said second depth is no more than about one-third of said tube wall thickness. 12. A method of repairing a part having a wall with a degraded area comprising the steps of; melting a localized area of the wall to a first depth sufficient to encompass said degraded area; allowing said localized area to cool and solidify; melting said localized area to a second depth, said second depth being substantially less than said first depth; allowing said localized area to cool and solidify. 13. The method of claim 12 wherein said second depth is about one-third of said first depth. 14. The method of claim 12, further comprising the step of maintaining a reactive cover gas over said localized area during the steps of melting. 15. A method of repairing a tube having a wall with a degraded area in said wall, the method comprising the steps of; applying first laser energy to a localized area of said wall, said first laser energy having a first frequen¬ cy, pulse duration and power level sufficient to melt said wall to a first depth in said wall; allowing said localized area to cool and solidify; applying second laser energy to said localized area, said second laser energy having a second frequency, pulse duration and power level sufficient to melt said wall to a second depth in said wall substantially less than said first depth; allowing said localized area to cool and solidify. 16. The method of claim 15, wherein said first frequency is substantially less than said second frequency. 17. The method of claim 16, wherein said first frequency is about 14 hertz and said second frequency is about 100 hertz. 18. The method of claim 15, wherein said first pulse duration is substantially longer than said second pulse duration. 19. The method of claim 18, wherein said first pulse duration is about .0076 seconds and said second pulse duration is about .0001 seconds. 20. The method of claim 15, further comprising the step of maintaining a reactive cover gas over said localized area during the steps of applying laser energy.

Inventors: KOZAK LARRY MICHAEL, NIELSEN STEEN ERIK, KOZAK, LARRY, MICHAEL, NIELSEN, STEEN, ERIK
Applicants: WESTINGHOUSE ELECTRIC CORP, WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1994
Main classification: B23P6
IPC codes: B23K26, B23P6, F28F11